"As I worked closely with President Bush, I would come to believe that sometimes he convinces himself to believe what suits his needs at the moment. It is not unlike a witness in court who does not want to implicate himself in wrongdoing, but is also concerned about perjuring himself. So he says, 'I do not recall.' The witness knows no one can get into his head and prove it is not true, so this seems like a much safer course than actually lying. Bush, similarly, has a way of falling back on the hazy memory defense to protect himself from potential political embarrassment. Bush rationalizes it as being acceptable because he is not stating unequivocally anything that could be proven false. If something later is uncovered to show what he knew, then he can deny lying in his own mind."
Scott McClellan. What Happened: Inside the Bush White House and Washington's Culture of Deceit
Umm... since when is pretending not to remember something to get out of saying it not "actually lying?" The idea that a falsehood is only "actually lying" if it can be proven without resorting to a psychic or a polygraph is a new one to me. But perhaps this is what lead the administration into trouble. If, even now, McClellan can't bring himself to characterize making ANY statement one knows to be false an "actual lie," it's easy to see how he went along to get along at the time.